In a sheet-making process, such as the manufacture of paper, sheet properties are commonly measured with sensors mounted on a scanner. The scanner traverses across the forming sheet back and forth while the paper sheet is moving in the direction perpendicular to the scanner's motion. A partially broken-away perspective view of a scanning system 100 is shown in FIG. 1. A scanner 102 is moved along a supporting frame which includes two beams 104 positioned one above a web 106 of material to be scanned and one below the web 106. The scanner 102 includes first and second members or heads 108, 110 which are moved back-and-forth along the beams 104 to scan the web 106 in the cross-machine direction (CD) or transversely to the web's direction of movement during manufacture. The web 106 of material is moved in the machine direction (MD) or x direction as indicated by the x axis of a coordinate system shown in FIG. 1 and the cross direction is in the y direction. A gap 112 is formed between the first and second heads 108, 110 with the web 106 of material to be scanned passing through the gap 112 for the scanning operation.
The web is sampled by one or more sensors moving along the traversing path to produce a continuous measurement which is processed to form a scanning measurement of a sheet property across the width of the sheet which is referred to as a “scan measurement”. Scan measurements consist of arrays of values that are accumulated over small CD widths called “databoxes” or over short periods of time called “time samples”, either of which may sometimes be referred to as “slices”. Ideally, the traversing paths are perfectly perpendicular to the machine direction and the variation of the entire sheet would be completely captured in a matrix where the MD variation is represented by the average of each scan measurement and the CD variation is represented by the shape of the scan measurement. In reality, the scan measurements obtained from a scanning sensor capture the sheet property variations along diagonal traversing paths. The measurement usually cannot be separated in MD and CD variations easily. The system of the present application enables MD and CD variations in sheet scan measurements to be quickly and effectively separated.